Non-contact capacitive datalink for a downhole assembly

ABSTRACT

Aspects of this invention include a downhole assembly having a non-contact, capacitive coupling including first and second transceivers deployed in corresponding first and second downhole tool members. The capacitive coupling is disposed to transfer electrical signals between the first and second transceivers. In one exemplary embodiment, the capacitive coupling is configured to transfer data and power between a substantially non-rotating tool member and a rotating tool member, for example, the shaft and blade housing in a steering tool. Exemplary embodiments of this invention provide a non-contact, high-speed data communication channel between first and second members of a downhole assembly. Moreover, exemplary embodiments of the invention also provide for simultaneous non-contact transmission of electrical power between the first and second tool members.

RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/897,597, filed Aug. 31, 2007, entitled “NON-CONTACT CAPACITIVEDATALINK FOR A DOWNHOLE ASSEMBLY.”

FIELD OF THE INVENTION

The present invention relates generally to downhole tools, for example,including directional drilling tools such as a steering tool and a mudmotor. More particularly, embodiments of this invention relate to adownhole assembly including a non-contact, capacitive coupling apparatusfor transmitting electrical power and/or data between first and secondmembers of the assembly.

BACKGROUND OF THE INVENTION

As is well-known in the industry, hydrocarbons are recovered fromsubterranean reservoirs by drilling a borehole (wellbore) into thereservoir. Such boreholes are commonly drilled using a rotating drillbit attached to the bottom of a drilling assembly (which is commonlyreferred to in the art as a bottom hole assembly or a BHA). The drillingassembly is commonly connected to the lower end of a drill stringincluding a long string of sections (joints) of drill pipe that areconnected end-to-end via threaded pipe connections. The drill bit,deployed at the lower end of the BHA, is rotated by rotating the drillstring from the surface and/or by a mud motor deployed in the BHA. Mudmotors are also commonly utilized with flexible, spoolable tubingcommonly referred to in the art as coiled tubing. During drilling adrilling fluid (referred to in the art as mud) is pumped downwardthrough the drill string (or coiled tubing) to provide lubrication andcooling of the drill bit. The drilling fluid exits the drilling assemblythrough ports located in the drill bit and travels upward, carryingdebris and cuttings, through the annular region between the drillingassembly and borehole wall.

In recent years, directional control of the borehole has becomeincreasingly important in the drilling of subterranean oil and gaswells, with a significant proportion of current drilling activityinvolving the drilling of deviated boreholes. Such deviated boreholesoften have complex profiles, including multiple doglegs and a horizontalsection that may be guided through thin, fault bearing strata, and aretypically utilized to more fully exploit hydrocarbon reservoirs.Deviated boreholes are often drilled using downhole steering tools, suchas two-dimensional and three-dimensional rotary steerable tools. Suchtools commonly include a plurality of independently operable blades (orforce application members) that are disposed to extend radially outwardfrom a tool housing into contact with the borehole wall. The directionof drilling may be controlled by controlling the magnitude and directionof the force or the magnitude and direction of the displacement appliedto the borehole wall. In rotary steerable tools, the housing istypically deployed about a rotatable shaft, which is coupled to thedrill string and disposed to transfer weight and torque from the surface(or from a mud motor) through the steering tool to the drill bitassembly.

Directional wells are also commonly drilled by causing a mud motor powersection to rotate the drill bit through a displaced axis while the drillstring remains stationary (non-rotating). The displaced axis may beachieved, for example, via a bent sub deployed above the mud motor oralternatively via a mud motor having a bent outer housing. The bent subor bent motor housing cause the direction of drilling to deviate (turn),resulting in a well section having a predetermined curvature (doglegseverity) in the direction of the bend. A drive shaft assembly deployedbelow the power section transmits downward force and power (rotarytorque) from the drill string and power section through a bearingassembly to the drill bit. Common drive shaft assemblies include arotatable shaft (mandrel) deployed in a housing.

The non-rotating sections (e.g., the above described housings) commonlyinclude MWD and/or LWD sensors, electronic components and controllers,and electrical actuators (e.g., solenoids used to control steeringblades). In the above described drilling assemblies a gap typicallyexists between the rotating and non-rotating sections (e.g., between theshaft and housing). Thus electrical power must be stored and/orgenerated in the non-rotating section or transferred across the gap fromthe rotating section to the non-rotating section. Moreover, in order toprovide electronic communication between the rotating and non-rotatingsections, data must also be transferred back and forth across the gap.

Techniques for transmitting electrical power and electronic data acrossthe gap between rotating and non-rotating tool sections are known in theart. For example, sealed slip rings are conventionally utilized. Whileslip rings are known to be commercially serviceable, failure of certainslip ring components is a known cause of downhole tool failure. Forexample, slip ring seals have been known to fail, which can result in aloss of communication with the tool and the need to trip out of theborehole. Loss of electrical contact between the slip ring contactmembers is also a known cause of tool failure.

Inductive coupling devices are also known for transferring power and/ordata between rotating and non rotating tool sections. For example, U.S.Pat. No. 6,540,032 to Krueger discloses an inductive coupling fortransferring power and data between rotating and non-rotating sectionsof a downhole drilling assembly. While inductive coupling devices areknown in commercial oilfield applications, there remains a need forimproved devices for non-contact transmission of data and electricalpower between tool sections. For example, inductive couplings tend tooccupy a large physical space and are typically expensive to fabricate(due to the use of a wound magnetic core). Inductive couplings also tendto exhibit low transmission efficiencies owing to the relatively largegap between transmitter and receiver. Owing to the demand for smallerdiameter and less expensive rotary steerable tools (and downhole toolsin general), there is a need for improved non-contact power and datatransmission devices.

SUMMARY OF THE INVENTION

The present invention addresses the need for improved non-contact powerand data transmission devices in downhole tools including downholedrilling assemblies. Aspects of this invention include a downholeassembly having a non-contact, capacitive coupling including first andsecond transceivers deployed in corresponding first and second downholetool members. The capacitive coupling is disposed to transfer electricalsignals between the first and second transceivers. In one exemplaryembodiment, the capacitive coupling is configured to transfer data andpower between a substantially non-rotating tool member and a rotatingtool member, for example, the shaft and blade housing in a steeringtool. In another exemplary embodiment, the capacitive coupling isdisposed to transfer data signals through a threaded pipe connection.Aspects of the invention typically further include electronic controlcircuitry for transmitting and receiving the electric signals.

Exemplary embodiments of the present invention may advantageouslyprovide several technical advantages. For example, exemplary embodimentsof this invention provide a non-contact, high-speed data communicationchannel between first and second members of a downhole assembly.Moreover, exemplary embodiments of the invention also provide forsimultaneous non-contact transmission of electrical power between thefirst and second tool members. Exemplary embodiments of the inventionalso tend to be relatively simple and inexpensive to manufacture ascompared to inductive couplings of the prior art. Exemplary capacitivecoupling embodiments also tend to advantageously be low mass and moreresistant to shock and vibration than prior art slip ring and inductivecoupling devices. In one exemplary embodiment, a capacitive couplingdevice in accordance with the invention may be advantageously configuredto transmit high-speed data signals through an electrical generator(alternator).

In one aspect the present invention includes a downhole assembly. Thedownhole assembly includes first and second downhole members and anon-contact, capacitive coupling device. The capacitive coupling deviceincludes first and second capacitively coupled transceivers and adielectric gap therebetween. The first transceiver is deployed in thefirst member and the second transceiver is deployed in the secondmember. The first and second transceivers are disposed to transfer anelectrical signal between the first and second members. In one exemplaryembodiment, the first member is a shaft and the second member is a toolhousing in which the shaft is deployed to rotate.

In another aspect this invention includes a downhole drilling assembly.The drilling assembly includes a shaft disposed to rotate in a toolhousing. A magnetic ring is deployed about the shaft and includes aplurality of circumferentially alternating magnets. An armature isdeployed in the housing substantially coaxially about the magnetic ring.The armature includes a plurality of radial windings such that rotationof the shaft in the housing produces AC electrical power. The assemblyfurther includes a non-contact capacitive coupling device having firstand second capacitively coupled transceivers with a dielectric gaptherebetween. The first transceiver is deployed in the shaft and thesecond transceiver is deployed in the tool housing. The capacitivecoupling device is disposed to transfer an electrical signal between theshaft and the tool housing.

In another aspect the present invention includes a threaded downholeconnector. The connector includes a first threaded member disposed to bethreadably connected with a second threaded member and a non-contact,capacitive coupling device including first and second capacitivelycoupled transceivers with a dielectric gap therebetween. The firsttransceiver is deployed in the first threaded member and the secondtransceiver is deployed in the second threaded member. The capacitivecoupling device is disposed to transfer an electrical signal between thefirst and second threaded members.

The foregoing has outlined rather broadly the features of the presentinvention in order that the detailed description of the invention thatfollows may be better understood. Additional features and advantages ofthe invention will be described hereinafter which form the subject ofthe claims of the invention. It should be appreciated by those skilledin the art that the conception and the specific embodiments disclosedmay be readily utilized as a basis for modifying or designing othermethods, structures, and encoding schemes for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a drilling rig on which exemplary embodiments of thepresent invention may be deployed.

FIG. 2 is a perspective view of one exemplary embodiment of the steeringtool shown on FIG. 1.

FIGS. 3A and 3B depict longitudinal and transverse cross sections of anexemplary non-contact, capacitive coupling device in accordance with thepresent invention deployed in the steering tool shown on FIG. 2.

FIG. 4 depicts a block diagram of an exemplary electronic circuit fortransmitting data and power across the capacitive coupling depicted inFIGS. 3A and 3B.

FIGS. 5 and 6 depict an alternative embodiment of a non-contact,capacitive coupling device in accordance with the invention deployed ina downhole threaded pipe connection.

FIG. 7 depicts a transverse cross section of a shaft driven downholealternator including a non-contact capacitive coupling device inaccordance with the invention.

DETAILED DESCRIPTION

Referring first to FIGS. 1 through 7, it will be understood thatfeatures or aspects of the embodiments illustrated may be shown fromvarious views. Where such features or aspects are common to particularviews, they are labeled using the same reference numeral. Thus, afeature or aspect labeled with a particular reference numeral on oneview in FIGS. 1 through 7 may be described herein with respect to thatreference numeral shown on other views.

FIG. 1 illustrates a drilling rig 10 suitable for utilizing exemplarydownhole drilling assembly (including steering tool and mud motor) andmethod embodiments of the present invention. In the exemplary embodimentshown on FIG. 1, a semisubmersible drilling platform 12 is positionedover an oil or gas formation (not shown) disposed below the sea floor16. A subsea conduit 18 extends from deck 20 of platform 12 to awellhead installation 22. The platform may include a derrick 26 and ahoisting apparatus 28 for raising and lowering the drill string 30,which, as shown, extends into borehole 40 and includes a drill bit 32and a steering tool 100 (such as a three-dimensional rotary steerabletool). In the exemplary embodiment shown, steering tool 100 includes aplurality of blades 150 (e.g., three) disposed to extend outward fromthe tool 100. The extension of the blades 150 into contact with theborehole wall is intended to eccenter the tool in the borehole, therebychanging an angle of approach of the drill bit 32 (which changes thedirection of drilling). Exemplary embodiments of steering tool 100further include hydraulic 130 and electronic 140 control modules (FIG.2) configured to control extension and retraction of the blades 150. Itwill be appreciate that control modules 130 and 140 typically includevarious electrical power consuming devices, such as, but not limited to,solenoid controllable valves, sensors (e.g., including accelerometers,pressure transducers, temperature sensors, rotation rate sensors, andthe like), and other electronic components (e.g., includingmicroprocessors, electronic memory, timers, and the like). The drillstring 30 may also include various electronic devices, e.g., including atelemetry system, additional sensors for sensing downholecharacteristics of the borehole and the surrounding formation, andmicrocontrollers disposed to be in electronic communication withelectronic control module 140. The invention is not limited in regardsto specific types or makes of electrical and/or electronic devices.

It will be understood by those of ordinary skill in the art that methodsand apparatuses in accordance with this invention are not limited to usewith a semisubmersible platform 12 as illustrated in FIG. 1. Thisinvention is equally well suited for use with any kind of subterraneandrilling operation, either offshore or onshore. While exemplaryembodiments of this invention are described below with respect to rotarysteerable embodiments. It will be appreciated that the invention is notlimited in this regard. For example, as described in more detail below,embodiments of the invention may also be utilized with mud motors (e.g.,deployed below the power section). Moreover, it will also be appreciatedthat the invention is not limited to downhole tool assemblies employingrotating and non rotating sections. For example, as described in moredetail below with respect to FIGS. 5 and 6, exemplary embodiments of theinvention may be utilized to provide a non-contact datalink betweensubstantially fixed downhole members.

Turning now to FIG. 2, one exemplary embodiment of steering tool 100from FIG. 1 is illustrated in perspective view. In the exemplaryembodiment shown, steering tool 100 is substantially cylindrical andincludes threaded ends 102 and 104 (threads not shown) for connectingwith other bottom hole assembly (BHA) components (e.g., connecting withthe drill bit at end 104 and upper BHA components at end 102). Thesteering tool 100 further includes a housing 110 and at least one blade150 deployed, for example, in a recess (not shown) in the housing 110.Control modules 130 and 140 are deployed in the housing 110. In general,the control modules 130 and 140 are configured for measuring andcontrolling the direction of drilling. Control modules 130 and 140 mayinclude substantially any devices known to those of skill in the art,such as those disclosed in U.S. Pat. No. 5,603,386 to Webster or U.S.Pat. No. 6,427,783 to Krueger et al.

To steer (i.e., change the direction of drilling), one or more of blades150 are extended into contact with the borehole wall. The steering tool100 is moved away from the center of the borehole by this operation,thereby altering the drilling path. It will be appreciated that the tool100 may also be moved back towards the borehole axis if it is alreadyeccentered. To facilitate controlled steering, the rotation rate of thehousing is desirably less than 0.1 rpm during drilling, although theinvention is not limited in this regard. By keeping the blades 150 in asubstantially fixed position with respect to the circumference of theborehole (i.e., by preventing rotation of the housing 110), it ispossible to steer the tool without constantly extending and retractingthe blades 150. Non-rotary steerable embodiments are thus typically onlyutilized in sliding mode. In rotary steerable embodiments, the tool 100is constructed so that the housing 110, which houses the blades 150,remains stationary, or substantially stationary, with respect to theborehole during directional drilling operations. The housing 110 istherefore constructed in a rotationally non-fixed (or floating) fashionwith respect to a shaft 115 (FIGS. 3A and 3B). The shaft 115 isconnected with the drill string and is disposed to transfer both torque(rotary power) and weight to the bit. As described above, the inventionis not limited to rotary steerable embodiments, nor even to embodimentshaving tool sections that rotate relative to one another.

The above described extension and/or retraction of the blades 150 isknown to consume electrical power. For example, in one commerciallyserviceable embodiment, the blades 150 are extended via hydraulicactuation with solenoid controllable valves being utilized to controlhydraulic fluid pressure at the individual blades. Electrically poweredhydraulic pumps have also been disclosed for controlling blade actuation(U.S. Pat. No. 6,609,579). Steering tool 100 typically further includeselectronics for sensing and controlling the position of each of theblades. Such electronics typically consume relatively little electricalpower as compared to the solenoids and/or electrical pumps describedabove, although the invention is not limited in regard to electric powerconsuming components deployed in the tool 100.

It will be appreciated that steering tool functionality isadvantageously enhanced by providing improved data transmission betweenhousing 110 and rotating shaft 115. For example, closed-loop steeringtechniques such as geo-steering techniques, commonly requirecommunication with LWD sensors deployed elsewhere in the drill string.Typical geo-steering applications make use of directional formationevaluation measurements (azimuthally sensitive LWD measurements) madevery low in the BHA, for example, in a rotating stabilizer located justabove the drill bit and/or even in the drill bit. To enable trueclosed-loop control, such directional formation evaluation measurementsare advantageously transmitted in substantially real time to electronicmodule 140. Electronic module 140 is also advantageously disposed inelectronic communication with a downhole telemetry system (e.g., a mudpulse telemetry system) for transmitting various steering tool dataup-hole. Such telemetry systems are typically deployed at the upper endof the BHA.

Turning now to FIGS. 3A and 3B, one exemplary embodiment of anon-contact, capacitive datalink 200 (FIG. 2) in accordance with thepresent invention is depicted in longitudinal (FIG. 3A) and transverse(FIG. 3B) cross section. Datalink 200 is disposed to transmit electricalpower (energy) and data in either direction across the gap 230 betweenthe housing 110 and shaft 115. In the exemplary embodiment shown,datalink 200 includes first and second thin-walled, cylindricaltransceivers 210 and 220 (also referred to herein as antenna plates).Transceiver 210 is deployed on an outer surface of the rotating shaft115, while transceiver 220 is deployed on an inner surface of thehousing 110. Transceivers 210 and 220 may be fabricated fromsubstantially any suitable electrically conductive material, e.g.,including conventional steels used to fabricate drill collars. In oneexemplary embodiment a gold-plated beryllium copper alloy may beadvantageous owing to its high electrical conductivity and corrosionresistance. Transceivers 210 and 220 are insulated from the main body ofthe shaft 115 and the main body of the tool housing 110, for example,via deployment in insulative housings 215 and 225. Housings 215 and 225may be fabricated from substantially any suitable insulative materialcapable of withstanding downhole conditions, for example, including PEEK(polyetheretherketone). As shown on FIGS. 3A and 3B, the insulativehousings 215 and 225 are disposed to electrically isolate thetransceivers 210 and 220 from the shaft 115 and housing 110. Suitableinsulators also advantageously tend to increase the dielectric constantof the gap 230 between the transceivers 210 and 220 (as described inmore detail below).

It will be appreciated by those of ordinary skill in the art thatdownhole tools must typically be designed to withstand shock levels inthe range of 1000 G on each axis and vibration levels of 50 G root meansquare. Such shock and vibration, typically due to engagement of thedrill bit with the formation, is known to cause eccentric rotation andaxial translation of the shaft 115 in housing 110. The exemplaryembodiment of the inventive capacitive coupling 200 shown on FIGS. 3Aand 3B is intended to accommodate expected downhole shock and vibration.In the exemplary embodiment shown, transceiver housings 215 and 225 (andtherefore transceivers 210 and 220) are disposed to translate/vibratetogether thereby maintaining gap 230 at a substantially constantthickness while simultaneously preventing relative rotation betweentransceiver housing 225 and tool housing 110.

With continued reference to FIGS. 3A and 3B, one or more bearings 255may be deployed between transceiver housings 215 and 225. It will beappreciated, that bearings 255 are disposed to maintain a substantiallyuniform gap 230 thickness during drilling (e.g., during the shocks andvibrations that are commonly encountered during drilling and duringrotation of the shaft 115 in the tool housing 110). While rollerbearings are depicted in the exemplary embodiment shown, the inventionis not limited in this regard. For example, a conventional journalbearing or bushing arrangement may also be utilized (journal bearingsare typically preferred since they tend to accommodate a very thin gap230). Notwithstanding, the invention is also expressly not limited tothe deployment of bearings of any kind between transceiver housings 215and 225. It will be appreciated that in certain embodiments conventionalbearing arrangements deployed elsewhere on the tool may providesufficient axial and lateral support to maintain the gap 230 at anapproximately constant thickness (especially if the datalink isimplemented in close proximity to the conventional bearing arrangement).The exemplary embodiment shown also includes an anti-rotation tab 245disposed to prevent relative rotation between the transceiver housing225 and tool housing 110. Again, the invention is not limited in thisregard.

In the exemplary embodiment shown on FIGS. 3A and 3B, spring members 240may be deployed between transceiver housing 225 and tool housing 110such that transceiver housing 225 accommodates eccentric rotation of theshaft 115. It will be understood that the invention is not limited toany particular spring configuration or number of spring members. Nor isthe invention even limited to the use springs or any other biasingmeans. In the exemplary embodiment shown, springs 240 are disposed toaccommodate lateral motion of the shaft 115 relative to the housing 110.The invention may alternatively and/or additionally include springsdisposed to accommodate axial motion of the shaft 115 relative to thehousing 110 for shock and vibration absorption.

Turning now to FIG. 4, a block diagram of exemplary control circuitryutilized for transmitting both electrical power and electronic databetween transceivers 210 and 220 is shown. The exemplary embodimentshown enables electronic data transfer in both directions; i.e., fromtransceiver 210 to transceiver 220 and from transceiver 220 totransceiver 210. The exemplary embodiment shown also enables electricalpower transmission from transceiver 210 to transceiver 220 (i.e., fromshaft 115 to tool housing 110), although the invention is not limited inthis regard. The invention may alternatively be configured to transmitpower from transceiver 220 to transceiver 210. Moreover, those ofordinary skill in the art will readily recognize that control circuitrymay be configured that enables power transmission in both directions(e.g., at distinct frequencies and/or during distinct time intervals).It will also be appreciated that the invention is not limited toembodiments in which both data and power may be transmitted through thecapacitive coupling device 200. Alternative embodiments may readily beconfigured for exclusive data transmission or exclusive powertransmission.

With continued reference to FIG. 4, the exemplary embodiment shownincludes first and second data transceiver circuits 410 electronicallyconnected to the corresponding transceivers 210 and 220. The exemplaryembodiment of transceiver circuits 410 depicted on FIG. 4 is configuredto provide bi-directional communication of conventional serialcommunication signals at 19,200 bits/sec, with each byte including 11bits (one start bit, nine data bits, and one stop bit). The inventionis, of course, not limited in regard to data communication rates and/orformats. It is expected that communication rates up to (and evenexceeding) 1 megabit/sec will be readily achievable using exemplaryembodiments of the invention. In the exemplary embodiment shown, datatransceiver circuits 410 each include a tuning circuit 412 (e.g., aconventional band pass filter) electronically coupled to transceivers210 and 220. In one advantageous embodiment, tuning circuit 412 has apass-band centered at about 1.23 MHz, although the invention is notlimited in this regard. Tuning circuit 412 is electronically connectedto amplifier filter 414 and antenna driver 416 which are in turnelectronically connected to a digital control circuit 418. The digitalcontrol circuit 418 is further electronically connected to a serialcommunication driver and protection circuit 420, which is in turnconnected to a communication bus 430 for communicating with other BHAcomponents.

When transmitting data, a data signal is received at the serialcommunication driver 420 from bus 430. The digital control circuit 418converts the digital signal to an analog signal which is used tomodulate a carrier frequency at the antenna driver 416. It will beunderstood that substantially any known modulation techniques may beutilized, for example, including amplitude, frequency, and phasemodulation. Conventional digital modulation schemes, for example,including QAM, DSL, ADSL, TDMA, FDMA, and the like, may also beutilized. In one advantageous embodiment, a carrier frequency of 1.23MHz is utilized, although the invention is not limited in this regard.Antenna driver 416 transmits the modulated data signal through thetuning circuit 412 to the corresponding transceiver 210, 220. The datasignal is received at the other transceiver 210, 220 and tuning circuit412 and amplified via amplifier filter 414. The digital control circuitconverts the modulated analog signal to a corresponding digital signal(e.g., a 19,200 bit per second, 5 volt signal) which is received by theserial communication driver 420.

As stated above, the exemplary embodiment shown is configured totransmit electrical power from the rotating shaft 115 to the toolhousing, i.e., from transceiver 210 to transceiver 220 on FIGS. 3A, 3B,and 4. As also stated above, the invention is not limited in thisregard. FIG. 4 shows a power source at 490. Power source 490 may includesubstantially any suitable downhole power source, e.g., including abattery pack, a mud-driven turbine alternator, and/or a shaft-driventurbine alternator. The power source 490 is electrically connected to apower control circuit 470 (e.g., a voltage regulator) which is in turnconnected to a power transmitting circuit 480. The power control circuitis typically further connected to (and provides power to) otherelectronic and electrical components, for example, including datatransceiver circuit 410. The power transmitting circuit includes ahigh-frequency generator 484 (e.g., 12.3 MHz in one advantageousembodiment) for converting electrical energy from the power controller470 to high-frequency AC. It will be appreciated that data and power maybe advantageously transmitted at mutually distinct frequencies, therebyenabling simultaneous data and power transmission. The oscillator 484 isconnected to an amplifier circuit 482 which is electrically connected totransceiver 210.

With continued reference to FIG. 4, transceiver 220 is electricallyconnected to a power receiver circuit 460, which receives thehigh-frequency electrical energy. In the exemplary embodiment shown,receiver circuit 460 includes a tuning network (tuned to the samefrequency as oscillator 484). A rectifier circuit 464 converts the highfrequency power to DC. A low-pass filter and bypass capacitors may beused with the rectifier circuit 464 to generate substantially noise-freeDC power. Power controller 470 receives the DC power from circuit 460and typically provides power to various electrical and electroniccomponents (e.g., including data transceiver circuit 410, solenoidcontrolled hydraulic valves, latch circuits, and various otherelectronic circuitry disposed in housing 110). Electrical power receivedat the controller may also optionally be utilized to charge rechargeablebatteries 472.

It will be understood by those of ordinary skill in the art that it isadvantageous to minimize the electrical impedance of the capacitivecoupling when it is used for power transmission applications (in orderto maximize power transmission capability and to minimize losses). Theimpedance of the coupling may be expressed mathematically, for example,as follows:

$\begin{matrix}{Z_{C} = {\frac{1}{{j\omega}\; C}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where Z_(C) represents the electrical impedance of the capacitivecoupling, j represents the imaginary number, √{square root over (−1)}, Crepresents the capacitance of the capacitive coupling, and ω representsthe transmitted frequency in radians (ω=2πf where f represents thefrequency). Those of ordinary skill will readily recognize that theimpedance Z_(C) is inversely proportional to the transmitted frequencyand the capacitance of the coupling. At any given frequency, theimpedance is inversely proportional to the capacitance. Thus, for powertransmission applications in which a low impedance is desirable, it istypically advantageous to maximize the capacitance of the inventivecoupling (e.g., to achieve a capacitance of greater than 100 pF).

The capacitance, C, of the capacitive coupling may be expressedmathematically as follows

$\begin{matrix}{C = \frac{{\kappa ɛ}_{0}A}{d}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

where κ represents the dielectric constant of the material in the gap(e.g., gap 230), ∈₀ represents the permittivity of free space (which isa constant having the value of approximately 8.55×10⁻¹² F/m), Arepresents the area of the transceivers 210, 220 on either side of thegap 230, and d represents the thickness of the gap. Those of ordinaryskill will readily recognize that the capacitance C is proportional to κand A, and inversely proportional to d. Thus, for power transmissionapplications, it may be advantageous to increase the area to thicknessratio (A/d) of the coupling as well as increase the dielectric constantκ of the medium in the gap.

In applications in which one transceiver rotates with respect to theother transceiver (e.g., the exemplary embodiment depicted on FIGS. 3Aand 3B), the gap is typically filled with a lubricating oil (althoughthe invention is not limited in this regard as the gap may alternativelybe evacuated). Suitable lubricating oils typically have a dielectricconstant of about 2 (about twice that of free space) at downholetemperatures and pressures. The dielectric constant of the gap may beadvantageously increased, for example, by utilizing a high dielectricconstant lubricating fluid or by employing high dielectric constantinsulators (e.g., insulative housings 215 and 225). PEEK has adielectric constant of about 3. Other higher dielectric constantmaterials may be advantageously utilized provided they are capable ofwithstanding demanding downhole conditions.

The surface area of the transceivers may be increased, for example, byincreasing the axial length of the cylinders. For rotary steerableembodiments, transceiver lengths of approximately 3 to 9 inches(resulting in a surface area of approximately 40 to 120 square inches)tend to be advantageous. It will be appreciated that while transceiversin accordance with the invention may occupy a relatively large area(e.g., of the inner surface of housing 110 and the outer surface ofshaft 115) they tend to occupy a relative small portion of the toolvolume. The thickness of the gap may be advantageously decreased, forexample, as described above, via the use of conventional journalbearings. In one exemplary embodiment that may be advantageouslyutilized for power transmission, the gap between the transceivers has athickness of less than about 0.1 inches (less than 2.5 mm).

It will be appreciated that data transmission across the capacitivecoupling typically requires the transmission of significantly lesselectrical energy than that of power transmission. For example, datatransmission typically only requires an electrical current on the orderof a few microamps or less. Useful power transmission, on the otherhand, typically involves transferring at least a milliamp of electricalcurrent. Thus it will be appreciated that exemplary embodiments of theinvention intended for data transmission only may be configureddifferently than embodiments that are intended for electrical powertransmission. For example, for data transmission only, it is notnecessarily advantageous to increase the capacitance of the capacitivecoupling. As a result, considerably smaller transceivers may be utilized(e.g., including an insulated wire as apposed to the plates shown onFIGS. 3A and 3B). Moreover, low current data signals may be transmittedacross a wider gap between the transceivers. Thus, for data transmissiononly, there is no need for journal bearings or other mechanicalarrangements intended to maintain a thin gap. The first and secondtransceivers also need not be axially overlapping for data transmission(whereas for power transmission the transceivers typically include arelatively large overlapping area as described above).

It will be appreciated that the use of bearings, springs, andanti-rotation mechanisms (e.g., bearings 225, springs 240, andanti-rotation tab 245 depicted on FIGS. 3A and 3B) is purely optional.In one exemplary embodiment of the invention, the capacitive couplingdoes not include bearings, springs, or any anti-rotation tab. Such acapacitive coupling has been found to be suitable for high-speed datatransmission and low power transmission applications (e.g., poweringelectronics components). Moreover, the capacitive coupling embodiment isinexpensive to fabricate and has been found to be highly robust andstable, advantageously providing for substantially maintenance free dataand low power transmission between shaft 115 and housing 110.

As stated above, the invention is not limited to rotary steerable oreven steering tool embodiments. Exemplary embodiments in accordance withthe invention may also be utilized, for example, in downhole motors (mudmotors). For example, conventional mud motors typically include abearing housing deployed below the power section, the bearing housingtypically including a mandrel deployed to rotate in an outer housing. Inone exemplary embodiment of the invention, a first transceiver may bedeployed on the outer surface of the mandrel and a second transceivermay be deployed on an inner surface of the housing (similar to thesteering tool embodiment depicted on FIGS. 3A and 3B).

Turning now to FIGS. 5 and 6, it will be appreciated that the inventionis also not limited to embodiments in which one transceiver is disposedto rotate with respect to the other. FIGS. 5 and 6 depict a threadeddownhole tool (pipe) connection including an alternative embodiment of acapacitive coupling 500 in accordance with present invention. Capacitivecoupling 500 is similar to coupling 200 described above in that itincludes first and second transceivers 510 and 520. In the exemplaryembodiment shown, transceiver 510 is deployed in pin end 540 andtransceiver 520 is deployed in box end 550. As shown in more detail onFIG. 6, transceivers 210 and 220 are deployed in correspondinginsulative housings 515 and 525. Housing 515 is deployed in a slot 542in an outer surface of the pin end 540 while housing 525 is deployed ina slot 552 in an inner surface of the box end 550. Transceivers 510 and520 are shown electrically connected to electrical wiring 532 and 534,such as conventional coaxial cable (the invention is not limited to anyparticular type of wiring). It will be appreciated that the invention isnot limited by the location of transceivers 510 and 520. For example,transceivers 510 and 520 may alternatively be located at 535 on FIG. 6.

In the exemplary embodiment shown, transceivers 510 and 520 includethin-walled cylindrical conductors. While the invention is not limitedin this regard, cylindrical transceivers advantageously eliminate theneed for achieving for particular angular orientation during make up. Assuch, the connection may be advantageously made up to substantially anydesirable torque and/or relative angular orientation. When the threadedconnection is made between pin end 540 and box end 550, the transceivers510 and 520 are brought into close proximity with one another therebyforming the capacitive coupling and enabling data transmission. It willbe appreciated that capacitive coupling 500 differs from capacitivecoupling 200 in that there is typically no lubricating fluid between thetransceivers 510 and 520. During make up of the connection, insulativehousings 515 and 525 may be brought into direct contact with oneanother. Housings 515 and 525 are typically slightly recessed tominimize compressive stresses during make up.

Exemplary embodiments of capacitive coupling 500 are typically suitablefor data transmission through a downhole pipe connection and may beadvantageously utilized for data communication between various BHA tools(e.g., including MWD, LWD, and steerable tool embodiments). It will beunderstood that capacitive couplings in accordance with the inventionmay also be utilized in substantially any downhole connection, forexample, those utilized in drill collars, pipes, cross-overs,stabilizers, bent-subs, vertical drilling tools, reamers, near bitstabilizers and drill bits. Exemplary embodiments of the invention mayalso be utilized in drill string communication systems similar to theIntelliPipe® system, which is available from IntelliServ® (a GrantPrideco Company). Implementation of exemplary capacitive couplingembodiments in accordance with the invention thus advantageously enablessubstantially real-time, high-speed, two-way communication among anetworked surface system (even an office computer) and substantially anydownhole tool.

With reference now to FIG. 7, a transverse cross section of a downholegenerator (alternator) 700 including a capacitive coupling device inaccordance with the invention is depicted. Downhole generator 700includes a magnetic ring 710 deployed about shaft 115. As shown,magnetic ring 710 includes a plurality of permanent magnets havingcircumferentially alternating magnetizations. While eight magnets (fourN and four S) are employed in the exemplary embodiment depicted, theinvention is by no means limited in this regard. Magnetic ring 710 istypically deployed in an insulative housing 715, which is disposed toelectrically insulate the magnets from the shaft 115. Downhole generator700 further includes a magnetic armature 720 having electricallyconductive windings deployed in the housing. Armature 720 is typicallydeployed in an electrically insulative housing 725 disposed to insulatethe armature 720 from the housing 110. Those of ordinary skill in theelectrical arts will readily recognize that rotation of shaft 115(including magnetic ring 710) in housing 110 (including the woundarmature 720) generates an alternating electric current in the windings.This AC power may be rectified via known means and used to power variouscomponents in the housing 110, for example, including electronics, latchcircuits, solenoids, electric motors, electric pumps, and the like. Therectified power may also be utilized to recharge a rechargeable batterypack. The invention is not limited in these regards.

Downhole generator 700 further includes a capacitive data-link disposedfor transmitting data between the rotating (e.g., shaft 115) andnon-rotating (e.g., housing 110) portions of the tool. In the exemplaryembodiment shown, magnets 710 and magnetic armature 720 are deployed oneither side of a dielectric gap 730 and are configured to function ascorresponding first and second transceivers. Although not shown on FIG.7, magnetic ring 710 and magnetic armature 720 are electricallyconnected to data transceiver circuits (e.g., circuits 410 shown on FIG.4) suitable for transmitting and receiving data signals through thecapacitive coupling. It will be appreciated that relativelyhigh-frequency electrical signals (e.g., about 1 MHz as described abovewith respect to FIG. 4) are typically (although not necessarily)employed for data transmission. Such high frequency signalsadvantageously reduce the impedance of the capacitive coupling andprevent interference with the AC power generated by rotation of themagnetic ring.

The incorporation of a capacitive datalink into downhole generator 700advantageously conserves valuable tool space while at the same timeproviding considerable electrical power for electrical componentsdeployed in the housing 110. The same tool space is advantageouslyutilized both to generate electrical power and transmit high-speed databetween the rotating and non-rotating tool components. At a shaftrotation rate of 200 rpm, exemplary embodiments of downhole generator700 are typically capable of producing a few Watts of electrical power.Such power generation advantageously obviates (or reduces) the need fordownhole battery packs. Data may be simultaneously transmitted (whileelectric power is being generated) back and forth through the generator700 (across the capacitive datalink). As described above with respect toFIG. 4, data transmission rates on the order of 1 megabit per second areexpected to be readily achievable (although the invention is not limitedin this regard).

Downhole generator 700 may also be advantageously utilized to measurethe rotation rate of shaft 115 relative to the housing 110. It will beappreciated that the electrical power produced by generator 700 has anAC frequency that is proportional to the rotation rate (theproportionality constant depending upon the number of magnets in themagnetic ring 710 and the number of windings in the armature 720). TheAC frequency may be determined by any of numerous electrical techniquesknown to those of ordinary skill in the electrical arts. For example,the analog signal produced by the generator may be converted to adigital signal (e.g., a square wave). A microprocessor may be readilyconfigured to determine the pulse frequency of the digital signal (e.g.,via detection of the rising edge of each pulse) and thus the rotationrate of the shaft. The measured rotation rate may be utilized by theprocessor to program the steering tool, for example, as disclosed incommonly assigned U.S. Pat. No. 7,222,681 and commonly assigned,co-pending U.S. Patent Publication 2005/0269082. Use of the downholegenerator 700 to measure the rotation rate of shaft 700 advantageouslyobviates (or provides redundancy to) other known means, e.g., includingHall-Effect sensors and magnets.

It will be appreciated that downhole generator 700 is not limited toembodiments in which magnetic ring 710 and magnetic armature 720function as transceivers in a capacitive datalink. In alternativeembodiments downhole generator 700 may also include distincttransceivers. For example, magnetic ring 710 may include a thin,conductive, non-magnetic plate deployed on its outer surface (facing thegap 730). Likewise, armature 720 may also include a thin, conductive,non-magnetic plate deployed on its inner surface (facing the gap). Theseplates, being insulated from the shaft 115 and housing 110 may beelectrically connected to data transceiver circuits and utilized totransmit data through generator 700. In another alternative embodiment,the windings deployed the armature 720 may be utilized as a transceiver.In such an embodiment, the magnetic ring 710 (or one of the abovedescribed plates) may be capacitively coupled directly to the windings.

It will be appreciated that capacitive data links in accordance with thepresent invention may be integrated into substantially any suitabledownhole tool structure having substantially any particular functionunrelated to the datalink (e.g., the downhole generator depicted in FIG.7). Alternative configurations will be apparent to those of skill in thedownhole arts.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalternations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims.

I claim:
 1. A downhole assembly comprising: first and second downholemembers; a non-contact, capacitive coupling device including first andsecond capacitively coupled transceivers and a dielectric gaptherebetween, the first transceiver deployed in the first member and thesecond transceiver deployed in the second member, the first and secondtransceivers disposed to transfer an electrical signal between the firstand second members; wherein the first and second transceivers aredeployed in corresponding first and second electrically insulativetransceiver housings, the first transceiver housing being deployed inthe first downhole member and the second transceiver housing beingdeployed in the second downhole member; and wherein a portion of each ofthe first and second insulative housings form the dielectric gap betweenthe transceivers.
 2. The downhole assembly of claim 1, wherein theelectrical signal comprises at least one member selected from the groupconsisting of electrical power and data.
 3. The downhole assembly ofclaim 1, wherein the first and second transceivers comprise first andsecond co-axial, electrically conductive cylinders.
 4. A threadeddownhole connector comprising: a first threaded member disposed to bethreadably connected with a second threaded member; a non-contact,capacitive coupling device including first and second capacitivelycoupled transceivers and a dielectric gap therebetween, the firsttransceiver deployed in the first threaded member and the secondtransceiver deployed in the second threaded member, the capacitivecoupling device disposed to transfer an electrical signal between thefirst and second threaded members; wherein the first and secondtransceivers are deployed in corresponding first and second electricallyinsulative transceiver housings, the first transceiver housing beingdeployed in the first threaded member and the second transceiver housingbeing deployed in the second threaded member; and wherein a portion ofeach of the first and second insulative housings form the dielectric gapbetween the transceivers when the connector is made up.
 5. The downholeconnector of claim 4, wherein the electrical signal comprises a datasignal.
 6. The downhole connector of claim 4, wherein the first andsecond transceivers comprise first and second co-axial, electricallyconductive cylinders.
 7. The downhole connector of claim 4, wherein thefirst and second transceivers are electrically insulated from thecorresponding first and second threaded members.
 8. The downholeconnector of claim 4, wherein the first and second insulative housingscontact one another when the connector is made up.
 9. The downholeconnector of claim 4, further comprising first and second electronictransceiver circuits, the first transceiver circuit electricallyconnected to the first transceiver and the second transceiver circuitelectrically connected to the second transceiver, the first and secondtransceiver circuits disposed to transfer the electrical signal betweenthe transceivers.
 10. The downhole connector of claim 9, wherein each ofthe first and second transceiver circuits comprises a data transceivercircuit, each of the data transceiver circuits disposed to transmit andreceive data between the transceivers.
 11. A string of downhole toolsconnected end to end, the string comprising the downhole connector ofclaim 4 deployed between adjacent ones of the tools, the string furtherincluding a plurality of tools being selected from the group consistingof drill collars, pipes, cross-overs, stabilizers, motors, bent-subs,MWD tools, LWD tools, steering tools, rotary steerable tools, verticaldrilling tools, reamers, near-bit stabilizers, and drill bits.
 12. Adownhole tool assembly comprising: first and second threaded membersconfigured to be threaded to one another; a first substantiallycylindrical transceiver deployed in the first threaded member, the firsttransceiver including a first electrically insulative housing and afirst electrically conductive transceiver element deployed therein, thefirst housing insulating the first transceiver element from the firstmember; a second substantially cylindrical transceiver deployed in thesecond threaded member, the second transceiver including a secondelectrically insulative housing and a second electrically conductivetransceiver element, the second housing insulating the secondtransceiver element from the second member; and wherein (i) the firsttransceiver is substantially coaxial in the second transceiver and (ii)the first and second insulative housings contact one another forming adielectric gap between the first and second transceiver elements whenthe first threaded member is threadably connected to the second threadedmember.